For years it was generally believed that high pressures of about 1000 psi were needed for molding thermoset sheet molding compounds in order to achieve proper flow. The use of such high pressures, however, was costly, requiring heavy duty presses and expensive forged steel tooling.
W. H. Englehart in "The First Successful Low Pressure SMC (SMC II) Application for a Class `A` Part," 33rd Annual Technical Conference, 1978, The Society of Plastics Industry, Inc. reported that SMC II, which was a low viscosity sheet molding compound marketed by Owens-Corning Fiberglas, could be used to mold a part having a class A surface at 500 psi. Since then low pressure sheet molding operations have been developed that operate at pressures as low as 200-300 psi.
Nevertheless, there is a continuing need for sheet molding compounds that can be molded at pressures below 100 psi, particularly with respect to unsaturated prepolymer/polyurethane sheet molding compounds such as polyester/polyurethane sheet molding compounds. The use of such low pressures would provide significant cost reductions in tooling requirements for molders. For example, the use of heavy duty presses could be avoided. Also, cast steel tooling rather than the more costly forged steel tooling could be used.
While the need for such low pressure sheet molding compounds has been clear, the solution to the problem of providing such compounds has not been clear. The present invention, however, provides such a solution.